Inspection apparatus and methods, substrates having metrology targets, lithographic system and device manufacturing method

ABSTRACT

Disclosed is an inspection apparatus for use in lithography. It comprises a support for a substrate carrying a plurality of metrology targets; an optical system for illuminating the targets under predetermined illumination conditions and for detecting predetermined portions of radiation diffracted by the targets under the illumination conditions; a processor arranged to calculate from said detected portions of diffracted radiation a measurement of asymmetry for a specific target; and a controller for causing the optical system and processor to measure asymmetry in at least two of said targets which have different known components of positional offset between structures and smaller sub-structures within a layer on the substrate and calculate from the results of said asymmetry measurements a measurement of a performance parameter of the lithographic process for structures of said smaller size. Also disclosed are substrates provided with a plurality of novel metrology targets formed by a lithographic process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application61/897,562, which was filed on Oct. 30, 2013 and which is incorporatedherein in its entirety by reference.

BACKGROUND

Field of the Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a “spectrum” from which aproperty of interest of the target can be determined. Determination ofthe property of interest may be performed by various techniques: e.g.,reconstruction of the target structure by iterative approaches such asrigorous coupled wave analysis or finite element methods; librarysearches; and principal component analysis.

In the known metrology technique, overlay measurement results areobtained by measuring the target twice under certain conditions, whileeither rotating the target or changing the illumination mode or imagingmode to obtain separately the −1^(st) and the +1^(st) diffraction orderintensities. Comparing these intensities for a given grating provides ameasurement of asymmetry in the grating, and asymmetry in an overlaygrating can be used as an indicator of overlay error.

Currently the overlay is deduced from targets which have a significantlylarger pitch than the product features under the assumption that bothare equal. Sub-segmented targets are sensitive to for instance lensaberrations, which cause a shift between the at-resolutionsubsegmentation and the larger overlay target grating pitch. Thereforethe effective accuracy of the overlay measurement is compromised.

A similar to overlay error between layers is mismatch betweenpopulations in a single layer, formed by different steps in a process.For example, the finest product features are nowadays formed bymultiple-patterning processes. It would be useful if the capabilities ofexisting metrology hardware could be extended to measurement of mismatchin double- and multiple-patterning processes. Again, however, the sizeof the product features is many times smaller than the resolution of themetrology hardware.

SUMMARY

It is desirable to provide a technique for overlay metrology which isless susceptible to inaccuracies of the type just mentioned, while usingif possible the existing metrology hardware.

It is desirable separately to provide a technique for mismatch metrologyusing the existing metrology hardware.

The invention in a first aspect provides an inspection apparatus formeasuring a property of a lithographic process, the apparatuscomprising: a support for a substrate carrying a plurality of metrologytargets comprising structures formed by the lithographic process; anoptical system for illuminating the plurality of targets underpredetermined illumination conditions and for detecting predeterminedportions of radiation diffracted by the targets under said illuminationconditions; a processor arranged to calculate from said detectedportions of diffracted radiation a measurement of asymmetry for aspecific target; and a controller for causing said optical system andprocessor to measure asymmetry in at least two of said targets whichhave different known components of positional offset between structuresand smaller sub-structures within a layer on the substrate and calculatefrom the results of said asymmetry measurements a measurement of aperformance parameter of the lithographic process for structures of saidsmaller size.

In one embodiment, said performance parameter is an overlay parameter ofthe lithographic process for structures of said smaller size and iscalculated by combining results of said asymmetry measurements withmeasurements of asymmetry in at least two overlay targets which havedifferent known components of positional offset between first and secondlayers on the substrate. Asymmetry may be measured in auxiliary targetshaving different known components of positional offset in each of thefirst and second layers.

In another embodiment, that may be applied in multiple-patterningprocesses, the controller is arranged to cause said optical system andprocessor to measure asymmetry in at least two of said targets whichhave different known components of positional offset between interleavedpopulations of sub-structures within the target and to calculate fromthe results of said asymmetry measurements a measurement of an overlayparameter of the lithographic process used to form said sub-structures.

In a second aspect, an embodiment of the present invention provides asubstrate provided with a plurality of metrology targets formed by alithographic process, each target comprising structures arranged torepeat with a spatial period in at least a first direction, wherein saidmetrology targets include: a plurality of overlay targets, at least someof said structures in each overlay target being replicated in first andsecond layers on said substrate and superimposed on one another andwherein each overlay target is formed with a positional offset betweenthe layers that is a combination of both known and unknown components,the known components being different for different targets; and aplurality of auxiliary targets, each auxiliary target comprisingsub-structures of a size several times smaller than said spatial period,wherein each auxiliary target is formed in one of said layers and isformed with a positional offset between the sub-structures andstructures that is a combination of both known and unknown components,the known components being different for different targets.

An embodiment of the present invention in the second aspect furtherprovides a patterning device (or pair of patterning deices) for use in alithographic process, the patterning device defining a pattern whichwhen applied to a substrate will produce a substrate according to thesecond aspect of an embodiment of the present invention, as set forthabove

An embodiment of the present invention in a third aspect provides asubstrate provided with a plurality of metrology targets formed by alithographic process, each target comprising structures arranged torepeat with a spatial period in at least a first direction, wherein saidmetrology targets include a plurality of targets each of which comprisessub-structures of a size several times smaller than said spatial period,wherein each target is formed with a positional offset between twointerleaved populations of sub-structures that is a combination of bothknown and unknown components, the known components being different fordifferent targets.

An embodiment of the present invention in the third aspect yet furtherprovides a pair of patterning devices for use in a lithographic process,the patterning devices defining patterns which when applied sequentiallyto a substrate will produce a substrate according to the third aspect ofan embodiment of the present invention, as set forth above.

An embodiment of the present invention in a further aspect provides amethod of measuring a performance parameter of a lithographic process,the method comprising the steps of: (a) performing said lithographicprocess to produce structures forming a plurality of metrology targetson a substrate, at least two of said targets having a positional offsetbetween structures and smaller sub-structures that is a combination ofboth known and unknown components, the known components of positionaloffset being different for different targets; (b) using the inspectionapparatus to measure asymmetry in at least two of said auxiliary targetshaving different known components of positional offset betweenstructures and smaller sub-structures within a layer on the substrate;and (c) calculating using the results of the asymmetry measurements madein step (b) a measurement overlay performance parameter of thelithographic process for structures of said smaller size.

An embodiment of the present invention in some embodiments can beimplemented using existing metrology apparatus such as a scatterometerAn embodiment of the present invention can be implemented in anautomated apparatus using modified software.

An embodiment of the present invention in the fourth aspect furtherprovides a computer program product comprising machine-readableinstructions for causing a processor to perform the step (c) of a methodas set forth above. The processor may be further programmed to controlan optical system and processor to perform the step (b) of the method.

An embodiment of the present invention yet further provides alithographic system comprising: a lithographic apparatus arranged totransfer a sequence of patterns from patterning devices onto a substratein an overlying manner; and

an inspection apparatus according to any of the aspects of an embodimentof the present inventions as set forth above, wherein the lithographicapparatus is arranged to use the calculated performance parameter fromthe inspection apparatus in applying said sequence of patterns tofurther substrates.

An embodiment of the present invention yet further provides a method ofmanufacturing devices wherein a sequence of device patterns is appliedto a series of substrates using a lithographic process, the methodincluding inspecting a plurality of metrology targets as part of orbeside said device patterns on at least one of said substrates using aninspection method as set forth above, and controlling the lithographicprocess for later substrates in accordance with the calculatedperformance parameter.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the invention;

FIGS. 3a-3d illustrate (a) a schematic diagram of a dark fieldscatterometer for use in measuring targets according to embodiments ofthe invention using a first pair of illumination apertures, (b) a detailof diffraction spectrum of a target grating for a given direction ofillumination (c) a second pair of illumination apertures providingfurther illumination modes in using the scatterometer for diffractionbased overlay measurements and (d) a third pair of illuminationapertures combining the first and second pair of apertures;

FIG. 4 depicts a known form of multiple grating target and an outline ofa measurement spot on a substrate;

FIG. 5 depicts an image of the target of FIG. 4 obtained in thescatterometer of FIG. 3;

FIG. 6 is a flowchart showing the steps of an overlay measurement methodusing the scatterometer of FIG. 3 and novel metrology targets, inaccordance with a first embodiment of the present invention;

FIG. 7 illustrates principles of overlay measurement applied inembodiments of the present invention;

FIGS. 8a and 8b illustrate a novel composite target in (a) plan view and(b) schematic cross-section having bias schemes and auxiliary targetsthat can be used in embodiments of the present invention;

FIGS. 9a and 9b illustrate part of a grating structure havingat-resolution features (a) in an ideal form and (b) with a displacementcaused by aberration in a lithographic step;

FIG. 10 shows in more detail an auxiliary component gratings in thenovel composite target for overlay metrology according to an embodimentof the invention;

FIG. 11 is an expanded portion of the flowchart of FIG. 6, showing howmeasurements of auxiliary component gratings in the target of FIG. 8 areused to produce corrected overlay measurement;

FIGS. 12 and 13 illustrated an alternative embodiment of the inventionusing large targets and using a pupil image sensor in the scatterometerof FIG. 3;

FIGS. 14(a) and 14(b) illustrate the phenomenon of mismatch instructures formed by multiple-patterning process;

FIG. 15 illustrates the form of component gratings in a novel compositetarget for measuring mismatch in structures formed bymultiple-patterning process according to an embodiment of the invention;and

FIG. 16 is a flowchart of a method of measuring mismatch in structuresformed by multiple-patterning process using the target of FIG. 15.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals, and others.Further, firmware, software, routines, instructions may be describedherein as performing certain actions. However, it should be appreciatedthat such descriptions are merely for convenience and that such actionsin fact result from computing devices, processors, controllers, or otherdevices executing the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner PW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g., mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the patterning device support (e.g., mask table) MT may be connected toa short-stroke actuator only, or may be fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features.

The depicted apparatus could be used in a variety of modes. In a scanmode, the patterning device support (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the patterning device support (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS. In scan mode, the maximum size of theexposure field limits the width (in the non-scanning direction) of thetarget portion in a single dynamic exposure, whereas the length of thescanning motion determines the height (in the scanning direction) of thetarget portion. Other types of lithographic apparatus and modes ofoperation are possible, as is well-known in the art. For example, a stepmode is known. In so-called “maskless” lithography, a programmablepatterning device is held stationary but with a changing pattern, andthe substrate table WT is moved or scanned.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Examples of known scatterometers include angle-resolved scatterometersof the type described in US2006033921A1 and US2010201963A1, which areincorporated by reference herein in their entireties. The targets usedby such scatterometers are relatively large, e.g., 40 μm by 40 μm,gratings and the measurement beam generates a spot that is smaller thanthe grating (i.e., the grating is underfilled). This simplifiesmathematical reconstruction of the target as it can be regarded asinfinite. In order to reduce the size of the targets, e.g., to 10 μm by10 μm or less, e.g., so they can be positioned in amongst productfeatures, rather than in the scribe lane, metrology has been proposed inwhich the grating is made smaller than the measurement spot (i.e., thegrating is overfilled). Typically such targets are measured using darkfield scatterometry in which the zeroth order of diffraction(corresponding to a specular reflection) is blocked, and only higherorders processed. Diffraction-based overlay using dark-field detectionof the diffraction orders enables overlay measurements on smallertargets. Examples of dark field metrology can be found in internationalpatent applications WO 2009/078708 and WO 2009/106279 which documentsare hereby incorporated by reference in their entirety. Furtherdevelopments of the technique have been described in published patentpublications US20110027704A, US20110043791A, US20120044470AUS20120123581A, US20130258310A and US20130271740A and in the U.S. patentapplications 61/652,552 and 61/803,673, which documents are herebyincorporated by reference in their entirety. These targets can besmaller than the illumination spot and may be surrounded by productstructures on a wafer. Multiple gratings can be measured in one image,using a composite grating target. The contents of all these applicationsare also incorporated herein by reference.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station and ameasurement station—between which the substrate tables can be exchanged.While one substrate on one substrate table is being exposed at theexposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. The preparatory steps may include mapping the surfacecontrol of the substrate using a level sensor LS and measuring theposition of alignment markers on the substrate using an alignment sensorAS.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

A metrology apparatus (scatterometer) suitable for use in embodiments ofthe invention is shown in FIG. 3(a). A grating target T and diffractedrays are illustrated in more detail in FIG. 3(b). More detail of theapparatus and variations in its forma and usage are provided in US2011027704 and other patent applications, mentioned above. The entirecontents of those applications are incorporated herein by reference. Thescatterometer may be a stand-alone device or incorporated in either thelithographic apparatus LA, e.g., at the measurement station, or thelithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. In thisapparatus, light emitted by source 11 (e.g., a xenon lamp) is directedonto substrate W via a beam splitter 15 by an optical system comprisinglenses 12, 14 and objective lens 16. These lenses are arranged in adouble sequence of a 4F arrangement. A different lens arrangement can beused, provided that it still provides a substrate image onto a detector,and simultaneously allows for access of an intermediate pupil-plane forspatial-frequency filtering. Therefore, the angular range at which theradiation is incident on the substrate can be selected by defining aspatial intensity distribution in a plane that presents the spatialspectrum of the substrate plane, here referred to as a (conjugate) pupilplane. In particular, this can be done by inserting an aperture plate 13of suitable form between lenses 12 and 14, in a plane which is aback-projected image of the objective lens pupil plane. In the exampleillustrated, aperture plate 13 has different forms, labeled 13N and 13S,allowing different illumination modes to be selected. The apertureplates in the present examples form various off-axis illumination modes.In the first illumination mode, aperture plate 13N provides off-axisillumination from a direction designated, for the sake of descriptiononly, as ‘north’. In a second illumination mode, aperture plate 13S isused to provide similar illumination, but from an opposite direction,labeled ‘south’. Other modes of illumination are possible by usingdifferent apertures. The rest of the pupil plane is desirably dark asany unnecessary light outside the desired illumination mode willinterfere with the desired measurement signals.

As shown in FIG. 3(b), grating target T is placed with substrate Wnormal to the optical axis O of objective lens 16. A ray of illuminationI impinging on target T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chain line+1 and double dot-chain line −1). It should be remembered that with anoverfilled small target grating, these rays are just one of manyparallel rays covering the area of the substrate including metrologytarget T and other features. Where a composite grating target isprovided, each individual grating within the target will give rise toits own diffraction spectrum. Since the aperture in plate 13 has afinite width (necessary to admit a useful quantity of light), theincident rays I will in fact occupy a range of angles, and thediffracted rays 0 and +1/−1 will be spread out somewhat. According tothe point spread function of a small target, each order +1 and −1 willbe further spread over a range of angles, not a single ideal ray asshown. Note that the grating pitches and illumination angles can bedesigned or adjusted so that the first order rays entering the objectivelens are closely aligned with the central optical axis. The raysillustrated in FIGS. 3(a) and 3(b) are shown somewhat off axis, purelyto enable them to be more easily distinguished in the diagram.

At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through beam splitter15. Returning to FIG. 3(a), both the first and second illumination modesare illustrated, by designating diametrically opposite apertures labeledas north (N) and south (S). When the incident ray I is from the northside of the optical axis, that is when the first illumination mode isapplied using aperture plate 13N, the +1 diffracted rays, which arelabeled +1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S the −1 diffractedrays (labeled −1(S)) are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g., a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for asymmetrymeasurement as well as for many measurement purposes such asreconstruction, which are not the subject of the present disclosure. Thefirst examples to be described will use the second measurement branch tomeasure asymmetry.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g., a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the −1 or +1 firstorder beam. The images captured by sensors 19 and 23 are output to imageprocessor and controller PU, the function of which will depend on theparticular type of measurements being performed. Note that the term‘image’ is used here in a broad sense. An image of the grating lines assuch will not be formed on sensor 23, if only one of the −1 and +1orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment of the invention,on-axis illumination of the targets is used, and an aperture stop withan off-axis aperture is used to pass substantially only one first orderof diffracted light to the sensor. (The apertures shown at 13 and 21 areeffectively swapped in that case.) In yet other embodiments, 2nd, 3rdand higher order beams (not shown in FIG. 3) can be used inmeasurements, instead of or in addition to the first order beams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Alternatively or in addition, a set of plates 13 could beprovided and swapped, to achieve the same effect. A programmableillumination device such as a deformable mirror array or transmissivespatial light modulator can be used also. Moving mirrors or prisms canbe used as another way to adjust the illumination mode.

As just explained in relation to aperture plate 13, the selection ofdiffraction orders for imaging can alternatively be achieved by alteringthe pupil-stop 21, or by substituting a pupil-stop having a differentpattern, or by replacing the fixed field stop with a programmablespatial light modulator. In that case the illumination side of themeasurement optical system can remain constant, while it is the imagingside that has first and second modes. In practice, there are manypossible types of measurement method, each with its own advantages anddisadvantages. In one method, the illumination mode is changed tomeasure the different orders. In another method, the imaging mode ischanged. In a third method, the illumination and imaging modes remainunchanged, but the target is rotated through 180 degrees. In each casethe desired effect is the same, namely to select first and secondportions of the non-zero order diffracted radiation which aresymmetrically opposite one another in the diffraction spectrum of thetarget.

While the optical system used for imaging in the present examples has awide entrance pupil which is restricted by the field stop 21, in otherembodiments or applications the entrance pupil size of the imagingsystem itself may be small enough to restrict to the desired order, andthus serve also as the field stop. Different aperture plates are shownin FIGS. 3(c) and (d) which can be used as described further below.

Typically, a target grating will be aligned with its grating linesrunning either north-south or east-west. That is to say, a grating willbe aligned in the X direction or the Y direction of the substrate W.Note that aperture plate 13N or 13S can only be used to measure gratingsoriented in one direction (X or Y depending on the set-up). Formeasurement of an orthogonal grating, rotation of the target through 90°and 270° might be implemented. More conveniently, however, illuminationfrom east or west is provided in the illumination optics, using theaperture plate 13E or 13W, shown in FIG. 3(c). The aperture plates 13Nto 13W can be separately formed and interchanged, or they may be asingle aperture plate which can be rotated by 90, 180 or 270 degrees. Asmentioned already, the off-axis apertures illustrated in FIG. 3(c) couldbe provided in field stop 21 instead of in illumination aperture plate13. In that case, the illumination would be on axis.

FIG. 3(d) shows a third pair of aperture plates that can be used tocombine the illumination modes of the first and second pairs. Apertureplate 13NW has apertures at north and east, while aperture plate 13SEhas apertures at south and west. Provided that cross-talk between thesedifferent diffraction signals is not too great, measurements of both Xand Y gratings can be performed without changing the illumination mode.A further variety of aperture plate 13Q will be illustrated in theexample of FIGS. 12 and 13.

Overlay Measurement Using Small Targets—Introduction

FIG. 4 depicts a composite grating target formed on a substrate Waccording to known practice. The composite target comprises fourindividual gratings 32 to 35 positioned closely together so that theywill all be within a measurement spot 31 formed by the illumination beamof the metrology apparatus. The four targets thus are all simultaneouslyilluminated and simultaneously imaged on sensors 19 and 23. In anexample dedicated to overlay measurement, gratings 32 to 35 arethemselves composite gratings formed by overlying gratings that arepatterned in different layers of the semi-conductor device formed onsubstrate W. Gratings 32 to 35 may have differently biased overlayoffsets in order to facilitate measurement of overlay between the layersin which the different parts of the composite gratings are formed.Gratings 32 to 35 may also differ in their orientation, as shown, so asto diffract incoming radiation in X and Y directions. In one example,gratings 32 and 34 are X-direction gratings with biases of +d, −d,respectively. This means that grating 32 has its overlying componentsarranged so that if they were both printed exactly at their nominallocations one of the components would be offset relative to the other bya distance d. Grating 34 has its components arranged so that, ifperfectly printed, there would be an offset of d but in the oppositedirection to the first grating and so on. Gratings 33 and 35 areY-direction gratings with offsets +d and −d respectively. While fourgratings are illustrated, another embodiment might require a largermatrix to obtain the desired accuracy. For example, a 3×3 array of ninecomposite gratings may have biases −4d, −3d, −2d, −d, 0, +d, +2d, +3d,+4d. Separate images of these gratings can be identified in the imagecaptured by sensor 23.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13NW or 13SE from FIG. 3(d). While the pupilplane image sensor 19 cannot resolve the different individual gratings32 to 35, the image sensor 23 can do so. The cross-hatched rectangle 40represents the field of the image on the sensor, within which theilluminated spot 31 on the substrate is imaged into a correspondingcircular area 41. Ideally the field is dark. Within this dark fieldimage, rectangular areas 42-45 represent the images of the individualgratings 32 to 35. If the gratings are located in product areas, productfeatures may also be visible in the periphery of this image field. Whileonly a single composite grating target is shown in the dark field imageof FIG. 5, in practice a semiconductor device or other product made bylithography may have many layers, and overlay measurements are desiredto be made between different pairs of layers. For each overlaymeasurement between pair of layers, one or more composite gratingtargets are required, and therefore other composite grating targets maybe present, within the image field. Image processor and controller PUprocesses these images using pattern recognition to identify theseparate images 42 to 45 of gratings 32 to 35.

Once the separate images of the gratings have been identified, theintensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter, and comparing the intensitiesreveals asymmetries that can be used as a measure of overlay. In anothertechnique for measuring asymmetry and hence overlay, the pupil planeimage sensor 19 is used. An example using this sensor will be describedlater with reference to FIGS. 12 and 13.

Measurement of At-Resolution Overlay

In modern lithographic processes, functional product features printed bythe lithographic apparatus may have very small dimensions, smaller thancan be resolved by the conventional metrology apparatus. Consequently,features in the gratings 32-35 of the metrology target are formed on alarger scale. As an example, the pitch of the metrology target may be inthe range 500 nm or 600 nm to 1000 nm or even 2000 nm. In other words,individual features (grating lines) would be 250 nm to 1000 nm in width.Product features formed at the resolution of the lithography tool mayhave dimensions less than 100 nm, for example less than 50 nm or evenless than 20 nm. These finer features are referred to commonly as“at-resolution” features, by reference to the resolving power of thepatterning system in the lithographic apparatus. It is known to formcoarse grating features of metrology gratings using at-resolutionfeatures, in order that effects of processing steps in the metrologygrating are not very different from effects in the product features.However, the metrology apparatus does not “see” these at-resolutionfeatures. (With respect to the metrology apparatus, they are“sub-resolution” features.)

While the metrology apparatus can measure the overlay error betweencoarse gratings to an accuracy of a few nanometers, such a coarsegrating is not representative of the actual product features. Themetrology targets are applied to the substrate by the same lithographicapparatus and process steps that form the functional product features,but the at-resolution features become subject to slightly differenterrors in their positioning than the coarser overlay grating features,for example due to aberrations in an optical projection system used toapply the pattern. The effect of this in current metrology apparatus isthat the measured overlay, while accurately representing overlay errorin the position of the coarse grating, does not accurately representoverlay in the finer, at-resolution features elsewhere on the samesubstrate. Since it is the at-resolution features that define theperformance of the functional end product, the result is that theaccuracy of the overlay measurement is not as relevant as one wouldlike.

The inventors have recognized that, by forming and measuring novelmetrology targets both with and without at-resolution features, themetrology apparatus can be used to obtain “at-resolution overlay”measurements, meaning overlay measurements that are more representativeof overlay between at-resolution product features elsewhere on thesubstrate. Before describing the novel targets and methods in detail, wewill present an overview of one example of the novel overlay measurementprocess.

FIG. 6 illustrates a method of measuring at-resolution overlay usingnovel targets. The method in this example is based on the methoddescribed in application US 2011027704 using the apparatus of FIGS. 3and 4. In principle, overlay error between the two layers containing thecomponent gratings 32 to 35 is measured through asymmetry of thegratings, as revealed by comparing their intensities in the +1 order and−1 order dark field images. At step S1, the substrate, for example asemiconductor wafer, is processed through the lithographic cell of FIG.2 one or more times, to create a structure including not only theoverlay gratings 32-35 but also auxiliary targets. The auxiliary targetscomprise gratings having coarse structures but also smaller-scale(at-resolution) sub-structures with programmed (known) offsets betweenthe at-resolution sub-structures and the coarse structures. Examples ofthese auxiliary gratings will be described in detail later. The overlaygratings 32-35 may comprise only coarse structures within the resolvingpower of the scatterometer, or may comprise at resolution features, butwithout different programmed offsets.

At S2, using the metrology apparatus of FIG. 3, an image of the gratings32 to 35 and the auxiliary gratings is obtained using only one of thefirst order diffracted beams (say −1). Then, whether by changing theillumination mode, or changing the imaging mode, or by rotatingsubstrate W by 180° in the field of view of the metrology apparatus, asecond image of the gratings using the other first order diffracted beam(+1) can be obtained (step S3). Consequently the +1 diffracted radiationis captured in the second image. It is a matter of design choice whetherall the gratings 32-35 and the auxiliary gratings can be captured ineach image, or whether the scatterometer and substrate need to be movedso as to capture the auxiliary gratings in one or more separate images.In either case, it is assumed that first and second images of all thecomponent gratings are captured via image sensor 23.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. Each grating will berepresented simply by an area of a certain intensity level. Theindividual grating lines will not be resolved, because only one of the+1 and −1 order diffracted radiation is present. In step S4, a region ofinterest (ROI) is carefully identified within the image of eachcomponent grating, from which intensity levels will be measured. This isdone because, particularly around the edges of the individual gratingimages, intensity values can be highly dependent on process variablessuch as resist thickness, composition, line shape, as well as edgeeffects generally.

Having identified the ROI for each individual grating and measured itsintensity, the asymmetry of the grating structure, and hence overlayerror, can then be determined. As described in the applications, this isdone by the image processor and controller PU in step S5 comparing theintensity values obtained for +1 and −1 orders for each grating 32-35 toidentify any difference in their intensity, and (S6) from knowledge ofthe overlay biases of the gratings to determine overlay error in thevicinity of the target T.

FIG. 7 illustrates the principle of calculating an overlay measurementfrom asymmetries in the intensity of different diffraction orders, usingthe method of FIG. 6. The horizontal axis represents overlay OVL, whilethe vertical axis represents an asymmetry signal A, obtained as adifference in intensity between the different diffraction orders of agiven target grating. The line 500 illustrates the (approximately)linear relationship between the asymmetry signal and the displacement ofone set of features (grating lines) in the overlay grating with respectto another. The scales of the axes are arbitrary and the slope of theline 500 need not be known in absolute terms. What is known is that theasymmetry signal goes to zero where the overlay is zero. Using biasedgratings, and knowledge of the biases, the unknown displacement can becalculated.

In the example, biased gratings with (programmed) offsets −d and +d areused. The fact that the offsets are equal and opposite is for simplicityonly. (In general, arbitrary offsets d1 and d2 can be envisaged.) In anideal case, where the target is printed perfectly, there is no othersource of displacement and the asymmetry in the gratings will be equaland opposite, as shown by open circles. In a real target, however, anunknown displacement Δd will also be present, which shifts the signalsto the positions shown by the solid circles. The asymmetry signalsobtained from the biased gratings are labeled A(−) and A(+). Knowing theoffsets −d and +d and knowing that the asymmetry should be zero whenoverlay is zero, the unknown displacement Δd can be calculated from theasymmetry signals to obtain a measurement of overlay error.

In the applications, mentioned above, various techniques are disclosedfor improving the quality of overlay measurements using the basic methodmentioned above. These techniques are explained in the applications, andwill not be explained here in further detail. They may be used incombination with the techniques newly disclosed in the presentapplication, which will now be described.

Returning to FIG. 6, in the present novel method, asymmetry is alsomeasured in the auxiliary gratings, so as to measure differences inposition between the coarse grating features and the at-resolutionfeatures on the substrate. In this way, the overlay measurement obtainedin step S6 is corrected to be more representative of the at-resolutionoverlay in the product features on the substrate. The principles andimplementation of this correction will now be described.

FIG. 8 shows a novel composite metrology target 520 for use in themethod of FIG. 6. The upper part (a) of the figure shows the target inplan view, while the lower part (b) shows it in cross-section. Thecross-section shows schematically substrate W and product layers L1 andL2. A real product will have many layers in practice. The compositetarget in this example comprises at its center a composite target 522that is identical to the set of component overlay gratings 32-35 used inthe known method. As seen in the cross-section, these targets havegrating features in both layers L1 and L2 (and which may includeat-resolution features). Either side of the target 522 are two auxiliarytargets 524 and 526. These auxiliary targets comprise gratings withcoarse features and at-resolution features, but formed only in onelayer. Thus target 524 comprises four auxiliary component gratings 32′to 35′ formed in layer L1, while target 526 comprises four auxiliarycomponent gratings 32″ to 35″ formed in layer L2.

Referring now to FIG. 9, we see part of a grating having “at-resolution”features that are similar in dimension to the functional productfeatures on the substrate, but are too small to be resolved individuallyby the scatterometer. FIG. 9 (a) shows in cross-section a small portionof a diffraction grating an overlay target such as the X-directiongrating 32′ in FIG. 9 (a). Specifically we see roughly one repeatingunit comprising a line-space pattern that is repeated with a knownperiodicity, to form the whole grating. The grating is formed inmaterials 600, 602 having different refractive indices, arranged in aperiodic pattern whose repeating unit comprises “line” regions 603 and“space” regions 604. The line-space pattern may in particular be formedby etching a pattern that is applied to a substrate using thelithographic apparatus of FIG. 1 or a similar apparatus. Thedesignations “line” and “space” in such a pattern are quite arbitrary.In fact, it will be noted that each “space” region 604 of the line isformed such that the material 600 is not uniformly absent, but is ratherpresent in a fine-pitch grating pattern comprising smaller lines 606 andspaces 608. Optionally, each “mark” region 603 may be formed such thatthe material 600 is not uniformly present, but is present in a similarfine pitch grating pattern. This fine pitch pattern may have aperiodicity in the Y direction, that is into the page, and is thereforenot visible in the cross-sections shown in FIG. 9. These finer lines andspaces are what is referred to herein as the “at-resolution” features,being at or close to the limit of resolution of the projection system inthe lithographic apparatus that will use them. They may also be referredto as “sub-resolution” features as far as the metrology apparatus(scatterometer) shown in FIG. 3 is concerned.

Ideally, the fine grating formed by lines 606 will be centered on thesame point 610 as the coarse grating. This point 610, averaged over allthe lines in the grating, may define a central reference position of thewhole target. Sub-segmented targets are sensitive to lens aberrations,however, in the process by which the target is formed. These aberrationscause a shift between the at-resolution features and the coarse gratingpitch.

FIG. 9 (b) shows the form of such a sub-segmented grating, similar tothe ideal form (a) but exhibiting a shift or mismatch between the coarsegrating pitch and the at-resolution features. This grating has becomeasymmetric due to a shift between the larger grating pitch and theat-resolution structures. A space 620 at one end of region 604 thesub-segmented space portion has become slightly narrower than space 622at the other end. The at-resolution grating therefore has a centralpoint at a position XAR which is not exactly coincident with the centralpoint X0 of the coarse overlay grating. A mismatch or shift Δdsrepresents the difference between X0 and XAR, and may be measured forexample in nanometers

Returning to FIG. 8, it can be seen that, while the overlay gratings32-35 have overlay offsets −d and +d programmed into them, auxiliarygratings 32′-35′ and 32″-35″ have programmed offsets in the positioningof at-resolution features with respect to the coarse grating structure.These offsets are labeled −ds and +ds in the X and Y direction gratings.The inventors have recognized that offsets between the at-resolutionfeatures and the coarse grating can be measured through asymmetrysignals in the same way as the main overlay can be measured. Bycombining the overlay measurement with auxiliary measurements made ineach layer, the corrected overlay measurement can be calculated in stepS6 of the method of FIG. 6.

FIG. 10 illustrates in detail the application of programmed offsets intwo auxiliary component gratings within one of the auxiliary targets,for example target 524. A schematic cross-section of a first componentgrating 32′ is shown at the top of the drawing, while a cross-section ofsecond component grating 34′ is shown at the bottom. In thecross-sections, as in FIG. 9, only one of the repeating units of theoverall pattern is shown, centered on a space region. Only threeat-resolution lines are shown, and shifts are exaggerated for clarity. Areal grating would have in the region of five to twenty at-resolutionlines and spaces in each space region of the larger pattern. In eachsegment there is both an unknown mismatch Δds caused by aberration orthe like during formation of the target, and a programmed (known) offset−ds or +ds. The unknown mismatch is equal (or assumed equal) for the twogratings. Again, the values of these offsets are chosen for simplicityto be equal and opposite, but the number and value of the programmedoffsets is a matter of choice. In practice, one would choose the offsetsto be positive and negative values of equal magnitudes. However, themethod to be described works with unequal magnitudes and with offsetsthat are both in the same direction. Similarly, the offset does not needto be either larger or smaller than the unknown mismatch. The exampleillustrated in FIG. 10 has offsets in opposite directions, but withmagnitudes less than the (unknown) mismatch Δd. Therefore the totaloffset is in the same direction both segments.

While the at-resolution features in this example comprise dense lines,the at-resolution features can take other forms, particularly in a casewhere the product features elsewhere, that are the real interest of theuser, have other forms. Thus the at-resolution features could be singlelines rather than gratings. They could be arrays of blocks instead oflines, or single blocks.

FIG. 11 shows more detail of the steps S5 and S6 that yield an overlaymeasurement corrected for the mismatch between coarse features andat-resolution features in the vicinity of the composite target 520.Referring also to FIG. 7 the programmed offsets −ds/+ds and unknownmismatch Δds will yield certain asymmetry signals A when measured withthe scatterometer of FIG. 3. In exactly the same way as thelayer-to-layer overlay Δd can be calculated from the measured asymmetrysignals and the known offsets, so the mismatch Δds in each layer L1, L2between the coarse grating and the at-resolution features can becalculated from measurement of the auxiliary targets 524, 526. Thus,step S5 includes auxiliary measurements S5′ on auxiliary target 524 andS5″ on auxiliary target 526. These are combined in step S6 with theoverlay measurement on target 522 to obtain a corrected overlaymeasurement Δd(AR) that is more representative. Various algorithms canbe used to calculate the corrected measurement. For example, one cancalculate explicitly the Δd and Δds values for each component targetbefore combining them. Alternatively, one could to firstly combine theasymmetry signals and then calculate a corrected overlay. One can applya more complicated analysis if desired, for example to bring inknowledge of the process and/or calibration data measured usingdifferent techniques.

The same measurements are repeated for the Y-direction overlay, and arealso performed for as many targets as desired, across the substrate. Thearrangement of the auxiliary gratings and overlay gratings in thecomposite target can be varied, for example mixing the overlay gratingsand auxiliary gratings rather than grouping them in separate compositetargets 522-526. Of course the number of component gratings in eachcomposite target can be varied also, and there need not be the samenumber of component gratings in the auxiliary targets as in the overlaygrating. In principle, an auxiliary grating may be provided in only oneof the layers, if correction of displacements in the other layer is notrequired.

Referring to FIGS. 12 and 13, the novel method can be applied not onlyto small targets with dark field scatterometry, but also with largetargets and angle-resolved scatterometry using the pupil plane imagesensor 19. For this example, a symmetric, segmented illumination profileillustrated at 13Q is used. Two diametrically opposite quadrants,labeled a and b, are bright in this aperture pattern (transparent),while the other two quadrants are dark (opaque). This type of apertureis known in scatterometry apparatus, from the published patentapplication US 20100201963. As seen at the center of FIG. 12, a targetgrating 732 is used that is underfilled by the illumination spot 31. Notshown in the drawing, this grating 732 is part of a larger set ofgratings forming component gratings of a composite target. By analogywith the example of FIG. 8, there may be component overlay gratings 732to 735 and auxiliary component gratings 732′ to 735′ and 732″ to 735″.

Whereas, in the example of FIGS. 4 to 6, detector 23 is used in an imageplane corresponding to the plane of substrate W, the method of FIGS. 12and 13 uses the detector 19 that is positioned in a plane conjugate witha pupil plane of objective 16. Detector 19 may be an image sensor, forexample a CCD camera sensor. Alternatively, individual point detectorsmay be deployed instead of image sensors. While the illumination patternprovided by aperture plate 13Q has bright quadrants labeled a and b atthe left hand side in FIG. 12, the diffraction pattern seen by sensor 19is represented at the right hand side. In this pattern, in addition tozero order reflections labeled a0 and b0 there are first orderdiffraction signals visible, labeled a−1, a+1, b−1 and b+1. Becauseother quadrants of the illumination aperture are dark, and moregenerally because the illumination pattern has 180° rotational symmetry,the diffraction orders a−1 and b+1 are “free” meaning that they do notoverlap with the zero order or higher order signals from other parts ofthe illumination aperture. This property of the segmented illuminationpattern can be exploited to obtain clear first order signals from adiffraction grating (overlay target) having a pitch which is half theminimum pitch that could be imaged if a conventional,circularly-symmetric illumination aperture were used. This diffractionpattern and the manner in which it can be exploited for scatterometry,are described in the known application US 20100201963.

FIG. 13 is a flowchart of the method of using the diffraction spectra ofFIG. 12 from the targets 732 etc. to obtain overlay measurementscorrected for at-resolution mismatch. The steps S11 to S15 correspondclosely to the steps S1 to S6 of the FIG. 6 method, and will not bedescribed in detail. The main difference is as follows. Recall that theFIG. 6 method obtains an asymmetry signal for grating 32 (for example)by comparing intensities of grating image 42 as seen in first and secondimages captured with the sensor 23. By contrast, the FIG. 13 methodobtains an asymmetry signal for grating 732 (for example) by comparingintensities of the +1 and −1 diffraction orders extracted from withinthe same diffraction spectrum on pupil image sensor 19.

Measurement of Overlay in Multiple-Patterned Targets

The techniques described above can be applied to use the knownscatterometer to measure mismatch between at-resolution features inother situations as well as in layer-to-layer overlay. A particularapplication is in so called double-patterning processes (generallymultiple-patterning), where successive lithographic patterning steps areused to produce a pattern of very small structures within a singleproduct layer, smaller than even the resolution of the patterningdevice. Techniques in this category include pitch-doubling, for exampleby litho-etch-litho-etch (LELE) and self-aligned dual-damascene in backend-of the line (BEOL) layers. It would be very useful to have ametrology technique to allow after-etch inspection and detection ofactual overlay shifts between the two respective process steps at actualdevice pattern resolution.

FIG. 14 (a) shows schematically a grating structure 800 formed by doublepatterning. Similarly to the grating of FIG. 9 (a), this gratingcomprises a coarse line-space pattern, in which space regions are filledwith sub-structures at a finer pitch. In the multiple-patterning processexample, the sub-structures are formed in one layer of the product, butnot in one patterning operation but in two or more steps. Thus, in thisexample, a first population of structures labeled A are interleaved witha second population of structures B, and the populations A and B areformed in different steps. While the placement of the populations A andB in FIG. 14 (a) is perfectly symmetrical, the structure 800′ shown inFIG. 14 (b) exhibits a certain positional offset or “mismatch”.Specifically, the population B structures are shifted relative to theirideal position by a mismatch amount labeled Δdp. The inventors haverecognized that mismatch between the at-resolution features and thecoarse grating can be measured through asymmetry signals in the same wayas overlay can be measured, if targets with programmed offsets areformed and measured.

FIG. 15 shows schematically a sub-segmented metrology target 820, wheretwo interleaved groups of sub-segmentation structures A and B formed ina pitch-doubling or other double patterning process. Two componentgratings 832 and 834 are formed, each having the general form of thegrating 800. Only six at-resolution lines are shown (three A and threeB), and shifts are exaggerated for clarity. A real grating would have inthe region of five to twenty or more at-resolution lines and spaces ineach space region of the larger pattern. In each grating 832, 834 thereis both an unknown positional offset (mismatch) Δdp caused by aberrationor processing effects the like during formation of the structure, and aprogrammed (known) positional offset −dp (in grating 832) or +dp (ingrating 834). The unknown mismatch is equal (or assumed equal) for thetwo gratings.

Again, the values of these offsets are chosen for simplicity to be equaland opposite, but the number and value of the programmed offsets is amatter of choice. In practice, one would choose the offsets to bepositive and negative values of equal magnitudes. However, the method tobe described works with unequal magnitudes and with offsets that areboth in the same direction. Similarly, the offset does not need to beeither larger or smaller than the unknown mismatch.

FIG. 16 is a flowchart of the method of using the novel target of FIG.15 to measure mismatch in a multiple-patterning process. The steps S21to S26 correspond closely with the steps S1 to S6 in the method of FIG.6. Similar considerations apply except that there is no layer-to-layeroverlay to be measured, only measurement of asymmetry of the componentgratings 832 and 834. The method can be adapted to use the pupil imagesensor 19 or another scatterometer, if desired. The principle ofcalculating the unknown mismatch using measured asymmetry signals andknown mismatch values (offsets) is the same as illustrated and describedabove with reference to FIG. 7.

Simulation indicates that even small overlay shift between the twopopulations can be detected using the known scatterometer hardware withsuitable targets. In the case of the modified overlay target, the numberof component gratings and the programmed mismatches can be varied.Overlay between layers can of course be measured in addition tomeasuring mismatch between populations within a layer. Mismatch can bemeasure in X and Y direction, if appropriate.

The techniques disclosed herein enable the design and use of small orlarge metrology targets to achieve great accuracy and repeatability ofoverlay measurements, and or measurements of mismatch in multiplepatterning processes. A particular benefit is that the existinghigh-throughput metrology hardware can be used to measure parameters ofat-resolution features, having sizes far below the resolution of themetrology apparatus optical system. The need for more time-consuming orexpensive metrology techniques (for example SEM) is reduced. Qualitycontrol in high-volume manufacture is enabled.

Numerous variations and modifications are possible, in addition to theones mentioned already above. In the examples of FIG. 8, the X and Ygratings with each bias value are side-by-side, though that is notessential. The X- and Y-direction gratings are interspersed with oneanother in an alternating pattern, so that different X gratings arediagonally spaced, not side-by-side with one another, and Y gratings arediagonally spaced, not side-by-side with one another. This arrangementmay help to reduce cross-talk between diffraction signals of thedifferent biased gratings. The whole arrangement thus allows a compacttarget design, with good performance. In the examples described above,all the gratings are square, and arranged in a square grid. In anotherembodiment these gratings may be placed slightly off the square grid, ormay be rectangular in shape in order to break the symmetry of thetarget. This may improve the accuracy & robustness of the patternrecognition algorithm that is used to find the targets in the imageseven further. Composite grating structures with elongate gratings aredescribed for example in published patent application US20120044470,mentioned above.

While the target structures described above are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target grating’ and ‘targetstructure’ as used herein do not require that the structure has beenprovided specifically for the measurement being performed. The terms“structure” and “sub-structure” are used to denote coarse (large-scale)and fine (smaller-scale) structural features, without intending thatthese features are entirely distinct from one another. Indeed, as isclearly explained in the examples, coarse structural features, such asthe lines and spaces of a grating, can be formed by collections of finersub-structures.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a methods of producing targets on a substrate,measuring targets on a substrate and/or analyzing measurements to obtaininformation about a lithographic process. This computer program may beexecuted for example within unit PU in the apparatus of FIG. 3 and/orthe control unit LACU of FIG. 2. There may also be provided a datastorage medium (e.g., semiconductor memory, magnetic or optical disk)having such a computer program stored therein. Where an existingmetrology apparatus, for example of the type shown in FIG. 3, is alreadyin production and/or in use, the invention can be implemented by theprovision of updated computer program products for causing a processorto perform the modified steps S4-S6 and so calculate overlay errorcorrected. The program may optionally be arranged to control the opticalsystem, substrate support and the like to perform automatically thesteps S2-S5, S12-S15, S22-S25 etc. for measurement of asymmetry on asuitable plurality of target structures.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. An inspection apparatus for measuring aproperty of a lithographic process, the apparatus comprising: a supportconfigured to support a substrate, the substrate carrying a plurality ofmetrology targets comprising structures and smaller sub-structuresformed by the lithographic process; an optical system configured toilluminate the plurality of metrology targets under predeterminedillumination conditions and to detect predetermined portions ofradiation diffracted by the plurality of metrology targets under theillumination conditions; a processor arranged to calculate, from thedetected portions of diffracted radiation, a measurement of asymmetryfor a specific target; and a controller configured to cause said opticalsystem and processor to measure asymmetry in at least two of theplurality of metrology targets, which have different known components ofpositional offset between the structures and the smaller sub-structureswithin a layer on the substrate and to calculate, from the results ofthe asymmetry measurements a measurement, of a performance parameter ofthe lithographic process for the smaller sub-structures.
 2. Theapparatus as claimed in claim 1, wherein the performance parameter is anoverlay parameter of the lithographic process for the smallersub-structures and is calculated by combining results of the asymmetrymeasurements with measurements of asymmetry in at least two overlaytargets which have different known components of positional offsetbetween first and second layers on the substrate.
 3. The apparatus asclaimed in claim 2, wherein the controller is configured to cause theoptical system and processor to measure asymmetry in at least twoauxiliary targets having different known components of positional offsetin each of the first and second layers.
 4. The apparatus as claimed inclaim 1, wherein the controller is configured to cause the opticalsystem and processor to measure asymmetry in at least two of themetrology targets which have different known components of positionaloffset between interleaved populations of sub-structures within themetrology target and to calculate, from the results of the asymmetrymeasurements, a measurement of an overlay parameter of the lithographicprocess used to form the sub-structures.
 5. The apparatus as claimed inclaim 1, wherein: the optical system is configured to form and detectimages using radiation diffracted by at least two of the metrologytargets simultaneously, different images using different portions of thediffracted radiation, and the processor is configured to identifyregions of interest in the detected images, each region of interestcorresponding to a specific one of the metrology targets and to processpixel values within the regions of interest to obtain the measurement ofasymmetry for each metrology target.
 6. A substrate comprising: aplurality of metrology targets formed by a lithographic process, eachtarget comprising structures arranged to repeat with a spatial period inat least a first direction, the metrology targets comprising: aplurality of overlay targets, at least some of the structures in eachoverlay target being replicated in first and second layers on thesubstrate and superimposed on one another and wherein each overlaytarget is formed with a positional offset between the layers that is acombination of both known and unknown components, the known componentsbeing different for different targets; and a plurality of auxiliarytargets, each auxiliary target comprising sub-structures of a sizeseveral times smaller than the spatial period, wherein each auxiliarytarget is formed in one of the layers and is formed with a positionaloffset between the sub-structures and structures that is a combinationof both known and unknown components, the known components beingdifferent for different metrology targets.
 7. The substrate as claimedin claim 6, wherein two or more overlay targets having different knownoffsets between layers are formed in close proximity with two or moreauxiliary targets having different positional offsets between thesub-structures and structures, so as to form a composite target, whileother such composite targets are distributed across the substrate. 8.The substrate as claimed in claim 6, wherein the auxiliary targetsinclude at least a first pair of targets having different knownpositional offsets that are formed in the first layer and at least asecond pair of targets having different positional offsets and that areformed in the second layer.
 9. A patterning device for use in alithographic process, the patterning device defining a pattern whichwhen applied to a substrate will produce a substrate comprising: aplurality of overlay targets, at least some of the structures in eachoverlay target being replicated in first and second layers on thesubstrate and superimposed on one another and wherein each overlaytarget is formed with a positional offset between the layers that is acombination of both known and unknown components, the known componentsbeing different for different targets; and a plurality of auxiliarytargets, each auxiliary target comprising sub-structures of a sizeseveral times smaller than the spatial period, wherein each auxiliarytarget is formed in one of the layers and is formed with a positionaloffset between the sub-structures and structures that is a combinationof both known and unknown components, the known components beingdifferent for different targets.
 10. A method of measuring a performanceparameter of a lithographic process, the method comprising: performingthe lithographic process to produce structures forming a plurality ofmetrology targets on a substrate, at least two of the targets having apositional offset between structures and smaller sub-structures that isa combination of both known and unknown components, the known componentsof positional offset being different for different targets; measuring,via the inspection apparatus, asymmetry in at least two of the auxiliarytargets having different known components of positional offset betweenstructures and smaller sub-structures within a layer on the substrate;and calculating a measurement overlay performance parameter of thelithographic process for structures of the smaller sub-structures usingthe results of the asymmetry measurements of the at least two of theauxiliary targets.
 11. The method as claimed in claim 10, wherein: thetargets formed in the performing the lithographic process are auxiliarytargets for at least two overlay targets, the overlay targets havingdifferent known components of positional offset between structures infirst and second layers on the substrate, and results of the asymmetrymeasurements made in the measuring are combined with measurements ofasymmetry in the at least two overlay targets to obtain a measurement ofan unknown component of the positional offset in the overlay target toobtain a representation of the overlay parameter of the lithographicprocess for structures of the smaller size.
 12. The method as claimed inclaim 11, wherein in the measuring at least two auxiliary targets havingdifferent known components of positional offset are measured in each ofthe first and second layers.
 13. The method as claimed in claim 10,wherein: the at least two of the targets are formed in the performingthe lithographic process to have different known components ofpositional offset between interleaved populations of sub-structureswithin a layer on the substrate, in a measurement of an overlayparameter of the lithographic process used to form the interleavedpopulations of sub-structures is calculated from the results of theasymmetry measurements made in the measuring of the asymmetry.
 14. Themethod as claimed in claim 10, wherein the performance of the opticalsystem is used to form and detect images using radiation diffracted byat least two of the metrology targets simultaneously, different imagesusing different portions of the diffracted radiation, and asymmetry ismeasured by identifying regions of interest in the detected images, eachregion of interest corresponding to a specific one of the targets, andprocessing pixel values from within the regions of interest to obtainthe measurement of asymmetry for each target.
 15. A computer programproduct comprising machine-readable instructions for causing a processorto perform a method comprising: performing the lithographic process toproduce structures forming a plurality of metrology targets on asubstrate, at least two of the targets having a positional offsetbetween structures and smaller sub-structures that is a combination ofboth known and unknown components, the known components of positionaloffset being different for different targets; measuring, via theinspection apparatus, asymmetry in at least two of the auxiliary targetshaving different known components of positional offset betweenstructures and smaller sub-structures within a layer on the substrate;and calculating a measurement overlay performance parameter of thelithographic process for structures of the smaller sub-structures usingthe results of the asymmetry measurements of the at least two of theauxiliary targets.
 16. A method of manufacturing devices wherein asequence of device patterns is applied to a series of substrates using alithographic process, the method comprising: inspecting a plurality ofmetrology targets as part of or beside the device patterns on at leastone of the substrates using an inspection method comprising: performingthe lithographic process to produce structures forming a plurality ofmetrology targets on a substrate, at least two of the targets having apositional offset between structures and smaller sub-structures that isa combination of both known and unknown components, the known componentsof positional offset being different for different targets; measuring,via the inspection apparatus, asymmetry in at least two of the auxiliarytargets having different known components of positional offset betweenstructures and smaller sub-structures within a layer on the substrate;and calculating a measurement overlay performance parameter of thelithographic process for structures of the smaller sub-structures usingthe results of the asymmetry measurements of the at least two of theauxiliary targets, and controlling the lithographic process for latersubstrates in accordance with the calculated performance parameter.